From: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Robert Haas <robertmhaas(at)gmail(dot)com>, Dimitri Fontaine <dim(at)hi-media(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: *_collapse_limit, geqo_threshold |
Date: | 2009-07-10 22:48:14 |
Message-ID: | 4A57C52E.8060200@cheapcomplexdevices.com |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> writes:
>> You do, but it's been pretty rare in my experience, and we're
>> considering alternatives which give a lot less flexibility that this.
>
> I dunno about "considering". We've already wasted vastly more time on
> this than it's worth. AFAIR there has never been one single user
> request for the ability to partially constrain join order. I think we
> should do an enable_join_ordering boolean and quit wasting brainpower on
> the issue.
I think I've found it useful in the past[1], but I also think we
already have a way to give postgres such hints using subselects
and "offset 0".
Instead of SAP-DB's
> select * from (t1 join t2 on <whatever>) join t3 on <whatever>;
ISTM we can already do
> select * from (select t1 join t2 on <whatever> offset 0) as a join t3 on <whatever>;
which seems like a reasonably way of hinting which parenthesis
can be reordered and which can't.
Would these new proposals give (guc's or syntax hacks) anything that
I can't already do?
[1] http://archives.postgresql.org/pgsql-performance/2007-12/msg00088.php
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